The present invention relates to the starting circuit of a self-excited inverter at the input side of a high-frequency switching power source using a MOSFET as the switching element. Conventionally, in a self-excited inverter circuit as is shown in FIG. 6, a trigger diode 5 is inserted between the coupling point of resistor 2 and capacitor 3 which are connected between both ends of DC power source 1 and the gate of the MOSFET 4 so that the capacitor 3 is made to oscillate by repeating the charging and discharging alternately as illustrated in FIG. 7(a) to output the pulse voltage Vp as is illustrated in FIG. 7(b) at the time of the discharge of the charged voltage Vc.sub.2 of capacitor 3, and this output is used to supply the gate voltage to the MOSFET 4 for starting; simultaneously with the start of the MOSFET 4, the charged voltage Vc.sub.2 of the capacitor 3 is discharged through diode 6, resistor 7 and the MOSFET 4 to prevent unnecessary restarting.
In the above example, numeral 8 represents the main transformer, 9 the primary input winding, 10 the secondary output winding, 11 the tertiary self-excited oscillation winding, 12 the saturation reactor, 13 and 14 the resistors.
In attempting to start the MOSFET 4 in a manner described above, the magnetic flux of the saturation reactor 12 is always set to one direction as indicated by the dotted line in FIG. 6, and so the alternate repetition of setting and resetting cannot be accomplished adequately. Needless to say, when the operation is started once, the saturation reactor 12 repeats the cycle of setting and resetting, though the saturation reactor continues to be saturated only in one direction, and some device is required to maintain stable operation. As stated previously, starting the circuit with the pulse from the trigger diode 5 is one of the methods for realizing stable operation, but this method is known to have the following disadvantages.
The trigger diode which is essential in the conventional circuit has its inherent and vital disadvantages. Firstly, for instance, the discharging voltage of the trigger diode is about 30 to 35V, which is not applicable to the switching power source whose input power source voltage is 30V-or less. Secondly, the manufacturers do not guarantee conformance to high industrial standards for communications equipment because of the construction of the trigger diode itself, and thus the manufacturers' guarantees are made conforming to their private standards. For example, the guaranteed operating temperature ranges from 0 to 85.degree. C.